Facts on CSOs
1. The CSO system performs a public health function to manage excess sewage overflows under extreme weather conditions. CSOs are permitted to operate at these times to relieve the sewerage network and prevent sewer flooding. Discharges through CSOs will be diluted and the bacterial loading impact from these will generally be minimal.
2. CSOs are regulated by Natural Resources Wales (NRW), the body that sets the permit conditions which must be met by CSOs. The number of spills from a CSO that are allowed is dependent upon the environmental sensitivity of the watercourse to which the discharge is made.
3. Dŵr Cymru Welsh Water (DCWW) has taken the most proactive position on CSO monitoring and their data is more extensive and of better quality than any of the other UK water companies. All CSOs in Wales were monitored by the end of 2020 (approx. 3000 assets). These data have been reported on Dŵr Cymru Welsh Water’s website by region and have been pro-actively shared with both Afonydd Cymru and the Rivers Trust. Welsh data is included on the Rivers Trust sewer map (https://experience.arcgis.com/experience/e834e261b53740eba2fe6736e37bbc7b?org=theriverstrust) which shows the geographical distribution of CSO spills across Wales. Despite this, the data for 2020 clearly shows that there are a significant number of CSO spills across Wales, including those that are operating outside of their permit conditions.